The efficiency of combustion of fuel-burning devices is a factor in the level of emissions of such devices. For example, when the fuel-burning device is an internal combustion (IC) engine such as in an automobile, the efficiency of combustion is a determinant of the level of release of greenhouse gases attainable by the automobile.
The efficiency of combustion of a liquid fuel in a fuel-burning device depends on the uniformity of the air/fuel mixture at the time of combustion. The uniformity of the air/fuel mixture may be increased by providing the fuel with viscoelastic properties, which may be accomplished by adding a polymer to the fuel. As the viscoelastic effectiveness of dilute polymer solutions is linear in polymer concentration and parabolic in molecular weight, a traditional method of improving the efficiency of combustion of a liquid fuel in a fuel-burning device is to add a high molecular weight polymer to the fuel.
That the polymer be of a high molecular weight is emphasized in the prior art. For example, in U.S. Pat. No. 5,906,665 (the '665 patent), high molecular weight polyisobutylene (PIB) was introduced into the fuel charge of an IC engine to provide viscoelastic properties to the fuel. The viscoelasticity imparted to the fuel results in a more uniform air/fuel mixture and, thus, more efficient combustion when compared to neat fuel. In the '665 patent, the extensional viscosity is shown to be proportional to cM(1+2 α), where c is the concentration, M is the viscosity average molecular weight of the polymer, and α is the exponent of M in the Mark-Houwink equation. Therefore, increasing the molecular weight of the polymer is taught as providing greater combustion efficiency.
Further, in Waters, P. F., Hadermann, A. F. and Trippe, J., “Solution Processing of Megadalton Molecular Weight Macromolecules,” Proceedings of the Second International Conference on Reactive Processing of Polymers, p. 11, J. T. Lindt, Ed., Univ. of Pittsburgh, Nov. 2-4, 1982, the antimisting effect of ultra high molecular weight macromolecules was examined in order to emphasize the significance of the contribution of the high molecular weights of these macromolecules to the viscoelastic properties of polymer solutions. The authors demonstrated that the higher the molecular weight of a polymer, the greater the antimisting effect of that polymer in solution; indeed, the measure of the effect increased parabolically with respect to its molecular weight. Since the antimisting effect of a polymer solution is a function of its viscoelasticity, it was concluded that an appropriate polymer of a higher molecular weight has a greater viscoelastic effect on a fuel.
In addition, in Waters, P. F., Hadermann, A. F. and Trippe, J., “The Effect of Molecular Weight of Additives on the Properties of Antimisting Fuels,” Division of Petroleum Chemistry Preprints, Vol. 28, No. 5, p. 1153, 186th National Meeting of the Am. Chem. Soc., Washington, D.C., 1983, the influence of the molecular weight on the height-at-break property of a column of polymer solution induced by a ductless siphon, the antimisting effectiveness, and, thus, the flammability suppression potential of PIB in isooctane were studied. The authors concluded that antimisting fuels containing ultra high molecular weight macromolecules show markedly superior antimisting effectiveness when compared to antimisting fuels containing the same concentration of lower molecular weight macromolecules. Therefore, it has been customary to select the highest molecular weight of an appropriate polymer to provide the desired viscoelastic properties to fuel.